The objective of this work is to elucidate the molecular mechanisms of the membrane transport of magnesium in Escherichia coli. The goals set for the present year include further work needed to distinguish the two distinct systems that we have recently discovered to be involved in the transport of magnesium in this organism. More specifically, experiments are being conducted to isolate mutants that are specifically blocked in system I or in system II. Mutants in system I are cobalt- resistant, but will grow on low levels of magnesium in the medium, because they possess system II. Procedures are now being devised for the isolation of mag II strains; such strains should be capable of growth on high, but not on low levels of magnesium.